People do not seem to be rushing to reply to you, gz. I took a look in The Art of Russian Cooking by Nina Nicolaieff and Nancy Phelan, published 1969. The kvass recipe they provide is made from black bread and yeast, plus raisins and sugar. "It is the great drink of the Russian peasant and is frequently mentioned in Rssian literature." Their recipe claims to be non-alcoholic, but it is refrigerated fairly soon.

This is the "normal" kvass but there are other drinks which might be regarded as variations - Yablochinkvas = cider; Grusheviikvas = perry, made from pears; Malinovoi = from raspberries.